The present invention generally relates to electrical conductors of the type formed on substrates. More particularly, this invention relates to a method for reducing the solderability of a thick-film conductor without significantly affecting its electrical, processing and mechanical properties, so that the shape and height of a solder connection between a surface-mount (SM) circuit device and the conductor can be controlled with a solderable pillar formed on the conductor.
Flip chips, ball grid arrays (BGAs), wire bond pads, chip resistors and chip capacitors are examples of surface-mount devices, i.e., discrete circuit devices mounted to the surface of a substrate, such as a printed circuit board (PCB), printed wiring board (PWB), flexible circuit, or a silicon, ceramic or insulated metal substrate. These devices rely on solder connections to both secure the chip to a substrate and electrically interconnect the device to conductors formed on the substrate. The size of a flip chip is generally on the order of a few millimeters per side, while bond pads, chip capacitors and resistors are typically smaller. As a result, the conductors required for surface-mount devices are narrow, e.g., line widths of about 0.5 millimeter or less, and typically spaced apart about 0.5 millimeter or less. While conductors can be formed by various methods such as plating and etching techniques, thick-film conductors are most often used in hybrid microcircuits.
Because of the small size of the solder connections, soldering a surface-mount device to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder using methods such as printing and electrodeposition. For smaller surface-mount devices, such as chip resistors and capacitors, the chip is soldered to its conductors by registering terminals formed on the chip with solder deposited on the conductors, and then reheating, or reflowing, the solder so as to form a xe2x80x9csolder columnxe2x80x9d that metallurgically adheres and electrically interconnects the chip to the conductors, yielding what will be referred to herein as a solder connection. When mounting flip chips and BGAs, solder is typically deposited on bond pads on the chip and then heated above its liquidus temperature to yield xe2x80x9csolder bumps.xe2x80x9d After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reflowing the solder to form solder connections.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the orientation of smaller surface-mount devices and the height of flip chip solder connections after soldering. As is well known in the art, smaller chips are prone to twisting and tilting during reflow as a result of the device floating on the surface of the molten solder, while controlling the height of flip chip solder connections after reflow is often necessary to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between a flip chip and its substrate, which may be termed the xe2x80x9cstand-off height,xe2x80x9d is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and underfill materials between the chip and its substrate.
The position and height of a solder column of a discrete component can be controlled by limiting the surface area over which the printed solder is allowed to reflow on a conductor. FIG. 1 is a longitudinal cross-sectional view of a conductor 12 on a circuit substrate 10, and shows the use of solder stops 14 to limit the flow of molten solder on the conductor 12. The solder stops 14 can be formed by a solder mask or printed dielectric, and extend widthwise across the surface 18 of the conductor 12. A solder bump 16 of a surface-mount device (not shown) is shown as being registered with the surface 18 of the conductor 12, as would be the case prior to reflow. The solder stops 14 delineate a rectangular-shaped area on the surface 18 of the conductor 12 over which the solder is able to flow during reflow. By properly locating the solder stops 14 on the conductor 12, the degree to which the molten solder can spread during reflow is controlled, which in turn determines the height of the solder connection and therefore the stand-off height of the device relative to the substrate 10.
Because the solder bump 16 is registered and soldered directly to the conductor 12, the conductor 12 must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor 12 through the formation of a strong metallurgical bond. In contrast, the solder stops 14 are intentionally formed of an unsolderable material, meaning that a tin, lead or indium-based solder will not adhere to the material for failure to form a strong metallurgical bond. While solder stops are widely used in the art, trends in the industry have complicated their ability to yield solder connections that exhibit adequate reliability for small discrete components, such as flip chips, chip capacitors and chip resistors. One trend is toward the use of low-melting, high-tin (e.g., 60Sn-40Pb) solders that are relatively brittle. Thermal cycle reliability problems can occur when a brittle solder solidifies against a solder stop used to contain the solder during reflow, and there is a tendency for fatigue fracturing to occur during thermal cycling at the junction between the conductor, solder and solder stop.
FIG. 2 illustrates an alternative to the use of solder stops in accordance with U.S. Pat. No. 5,926,732 to Coapman et al., assigned to the assignee of the present invention. Coapman et al. disclose forming a conductor 112 of an unsolderable thick-film conductive material, and then forming a solderable pillar 114 on the surface 118 of the conductor 112, to which a solder bump 116 of a surface-mount device (not shown) is then registered. According to Coapman et al., when the solder bump 116 is reflowed, the molten solder alloy will coalesce on the pillar 114, such that the pillar 114 determines the placement and height of the resulting solder connection on the conductor 112 following reflow. The pillar 114 can be appropriately sized and shaped to ensure that the solder bump 116 will form a connection having an adequate height for allowing an underfill material to flow between the device and the substrate 110 on which the conductor 112 is formed. Solder connections formed in accordance with Coapman et al. have been shown to exhibit significantly improved reliability for small discrete components having fine pitch terminal patterns.
A complication with implemting the teachings of Coapman et al. has been the limited availability of thick -film conductive materials that are unsolderable, i.e., to which a tin, lead or indium-based solder will not adhere to form a strong metallurgical bond. In hybird circuits, thick-film conductors based on silver-platinum and silver-palladium alloys are often preferred for their electrical, mechanical and processing characteristics. However, most of these alloys form strong metallurgical bonds with solder alloys, and therefore cannot be used with the teachings of Coapman et al. Nontheless, it would be desiable if thick-film conductive materials having the desirable electrical, mechanical and processing characteristics of silver-platinum and silver-palladium alloys could be used with the teachings of Coapman et al.
The present invention provides a conductor, a method for forming the conductor, and a method for attaching a surface-mount circuit device, such as a chip capacitor, chip resistor or bond pad, to the conductor with solder connections. The invention utilizes the pillars taught by Coapman et al. to form solder connections that are characterized as being accurately located on the conductor and having a shape that achieves an adequate stand-off height for the device, promotes stress relief during thermal cycling, and reduces the likelihood that the device will twist and tilt during reflow.
According to this invention, the thick-film conductor is formed of a conductive ink that would normally produce a solderable conductor, but is rendered unsolderable by additions of a fine particulate inorganic material. The inorganic material is present as a fine dispersion and in a sufficient quantity within the conductor to inhibit the formation of a strong metallurgical bond between the conductor and a tin, lead or indium-based solder alloy, but not in quantities that significantly affect the electrical, mechanical and processing characteristics of the conductor. For this reason, the inorganic material is preferably processed to include a surfactant coating that inhibits agglomeration, and is limited to about 4 weight percent of total solids of the conductive ink. After printing, drying and firing using any suitable methods, the thick-film conductive ink yields a thick-film electrical conductor having reduced solderability as compared to a thick-film electrical conductor formed only of the conductive ink with no additives.
In view of the above, the present invention enables the use of thick-film conductive inks based on solderable metals, including commercially-available silver, silver-platinum and silver-palladium alloys, to form unsolderable conductors having desirable electrical, mechanical and processing characteristics. A conductive ink can be rendered sufficiently unsolderable so that, when a solderable pillar is formed on an unsolderable conductor formed of the ink and a solder bump is registered and reflowed on the pillar, the molten solder alloy will coalesce only on the pillar. As a result, the pillar determines the placement and height of the resulting solder connection on the conductor following reflow. An appropriately sized and shaped pillar on the unsolderable conductor can ensure that the solder bump will form a connection having an adequate height to allow an underfill material to flow between a surface mount device and substrate. The invention achieves the above without the use of conventional solder stops that overlie the conductor. Accordingly, conductors and solder connections of this invention are not susceptible to the same problems with contamination, underfill voiding and stress raisers that can reduce the reliability and durability of prior art connections made using solder stops. In addition, the increased chip height made possible with this invention as a result of tighter control of solder during reflow reduces the mechanical stresses on the chip connection and increases the reliability of the attachment.
Other objects and advantages of this invention will be better appreciated from the following detailed description.